1. Field of the Invention
This invention relates generally to sun tracking apparatus including diurnal tracking with seasonal adjustment.
2. Discussion of the Prior Art
Despite the urgent need for a cost effective method of using solar energy to replace more conventional energy sources, progress toward this goal has been very slow. In order for solar energy to supply a significant proportion of the energy required by mankind, it is generally believed that it will be necessary to provide efficient means for converting solar energy directly into electrical energy, such as by use of solar cells, or into a more managable form of heat, such as by use of solar collectors for heating a heat transfer medium. While inexpensive stationary solar cells and collectors are useful in some applications, optimal utilization of solar energy appears to require some type of apparatus for tracking the movement of the sun preferably both diurnally (throughout the day light hours) and seasonally so that the solar cells or the collectors are always arranged perpendicularly to the sun line, defined as the line between a point on the earth and the center of the sun.
Numerous attempts have been made to produce a sun tracking system which is capable of maintaining the desired orientation of a panel of solar cells or collectors at all times throughout the day and year as discussed above but these efforts have yet to yield a sufficiently effective low cost sun tracker. For example, the patent to Toulmin, Jr. (U.S. Pat. No. 3,070,643) discloses a closed loop servo system for continuously pointing a solar cell directly toward the sun by sensing the sun's position and selectively driving the solar cell support about one or the other of a pair of axis. Devices of this type (termed heliostats) are well known but suffer from a number of drawbacks especially including high cost and complexity. For example, the Toulmin, Jr. patent discloses a complicated gearing system with a single drive motor and an electrically operated clutch to permit selective dual axis drive. Other systems such as illustrated in U.S. Pat. Nos. 3,998,206 and 3,996,917 employ separate drive motors for obtaining dual axis movement. While useful for the purposes disclosed these closed loop servo systems (that is, systems employing sun position sensors for controlling the timing and amount of tracking movement by the servo system) are costly to install and are subject to mechanical or electrical breakdown which adversely affects their reliability. Moreover, during periods of partial cloud cover, the ability of such systems to track the sun may be impaired; thus, requiring the system to include an open loop servo capability (that is an ability to track the positions of the sun without using sun position sensors) such as is disclosed in U.S. Pat. No. 4,031,385. Obviously the use of both an open loop and closed loop servo control in the same system adds significantly to the cost of initial installation and maintenance of a sun tracking apparatus.
Various types of open loop sun tracking systems have been disclosed such as illustrated in U.S. Pat. Nos. 787,145 and 4,011,854 in which mechanical or electrical timers are used to control the movement of the tracker without the aid of sun position sensors. Both of the trackers illustrated in these patents permit seasonal adjustment but provide only single axis rotation during each day. While single axis rotation permits acceptable tracking of the sun's position during certain times of the year and at certain points on the earth's surface, at other times and locations, single axis rotation during the day is not sufficiently accurate in terms of maintaining the desired orientation of the solar cells or collectors throughout the entire day.
Some open loop trackers are known (such as illustrated in U.S. Pat. No. 820,127) which provide daily dual axis tracking using a single motor drive. However, such systems have not generally been designed to reproduce the complicated changes in the celestial path which the sun follows diurnally and seasonally. For example, U.S. Pat. No. 820,127 discloses an open loop dual axis sun tracker wherein rotation about a horizontal axis and a vertical axis is effected by a single motor through a constant ratio drive train interconnecting the vertical and horizontal rotational supports. Obviously, a system of this type cannot accommodate seasonal changes without complicated modifications of the drive train.